The present invention relates to a fixing device for use in electrophotographic image forming apparatuses such as copying machines, laser printers and facsimile, and an image forming apparatus using the fixing device.
Conventional fixing devices include those detecting temperature on the surface of a heating roller by a noncontact temperature sensor (see JP 06-19367 A). Generally, the temperature sensor is covered with a cover material made of resin and located on an opposite side of the heating roller for protection of the sensor.
In the conventional fixing device, however, since the cover member made of resin is low in thermal conductivity and heat reflectivity, it is hard to conduct heat from the heating roller to the cover member. This makes it difficult to warm up the cover member, and therefore, to transfer the heat from the heating roller to the temperature sensor via the cover member. Thus, the temperature rise slows down in the temperature sensor. Moreover, the degree of heat reflection of the cover member is low. Therefore, it is difficult for the cover member to reflect radiant heat coming from the heating roller, and the radiant heat is not effectively conducted to the temperature sensor.
Thus, the temperature sensor has poor response to the temperature rise on the surface of the heating roller. This makes it impossible to reduce time taken for an operation which increases the temperature on the surface of the heating roller to a fixable temperature (hereinbelow referred to as warm-up).
More particularly, if the warm-up time is reduced and the temperature of the heating roller rises in a short period of time, then the heating roller suffers, for example, excessive temperature rise. This is because the temperature sensor is unable to detect the temperature of the heating roller with immediate response. When the temperature excessively rises in the heating roller, it becomes impossible to guarantee fixing quality of the recording member, durability of the heating roller and safety of the heating roller in terms of ignition prevention.